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Topic: Wittig reagent - cis or trans?  (Read 1373 times)

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Offline maverick1996

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Wittig reagent - cis or trans?
« on: April 18, 2016, 06:56:36 AM »
Hey guy!

When I react a ylid with an aldehyde, the product is an alkene and I just assumed that a trans alkene would be more favourable due to less steric repulsion etc.

But I am doing some questions, the specific question is EtCHO with Ph3P=CHCH2CH2CH3 (sorry about not formatting this properly) and again I assumed the trans alkene is the majority product but the answers say 'EtCH=CHCH2CH2CH3, 90% cis-isomer'

Can somebody help me out and explain why the cis alkene is the majority product?

Thank you

Offline Alwin Kristen

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Re: Wittig reagent - cis or trans?
« Reply #1 on: April 18, 2016, 10:42:51 AM »
Stereoselectivity of Wittig-reaction depends of the ylid. With stabilized ylid (delocalized negative charge) reaction is E (trans) selective and with unstabilized ylid its Z (cis) selective. I have attached a picture from J. Claydens book; Organic Chemistry, to clarify Z selectivity.
Let us learn to dream, we then might find the truth. - F. A. Kekulé

Offline maverick1996

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Re: Wittig reagent - cis or trans?
« Reply #2 on: April 18, 2016, 12:59:03 PM »
Stereoselectivity of Wittig-reaction depends of the ylid. With stabilized ylid (delocalized negative charge) reaction is E (trans) selective and with unstabilized ylid its Z (cis) selective. I have attached a picture from J. Claydens book; Organic Chemistry, to clarify Z selectivity.

Thank you

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